In order to ensure the reliable and effective transmission of various kinds of information in a channel, it is usual to adopt an error control encoding technology in a modern digital communication system. The error controlling method in the digital communication is basically divided into two types, that is, an automatic repeat request (ARQ) system and a forward error correction (FEC) system. The advantage of the FEC communication system is that only one one-way-channel is required and the system has the high transmitting efficiency. However, there are some disadvantages in the FEC system, that is, the reliability of the FEC communication system is not high because the mistaken information will be sent to subscriber when some mistakes occur in decoding; and it is necessary to use the long code and select the code group having the high error correcting ability in order to obtain the high system reliability, causing the complex decoding circuit and the high manufacturing cost.
Compared with the FEC communication system, the ARQ communication system has the simple structure and the high reliability, however, it is necessary to provide a reverse channel and the transmitting efficiency will be very low when the channel condition becomes poor (i.e. the error-rate is too high) because the system is often in a requesting state.
In view of the respective advantages and disadvantages of the FEC and ARQ systems, a hybrid ARQ communication system is constituted by combining the FEC and ARQ systems properly. In the hybrid ARQ system, the ARQ mechanism is combined with FEC, so that both reliability and transmitting efficiency of the system can be increased. In a hybrid ARQ solution realized by Chase combining of the prior art, the acknowledgement (ACK)/negative acknowledgement (NACK) indications will be fed back from the receiver of the opposite communication terminal to the transmitter of the local terminal according to the CRC checking result so as to determine whether the retransmission is necessary. If the retransmission is necessary, the whole block of data transmitted before must be transmitted again. In the receiver, the block of data transmitted again is weighted according to the signal-to-noise ratio during correspondingly transmission interval and combined together again. Then the combined block of data is soft-input to the decoder of the channel for decoding in channel. The famous Chase combining method above-mentioned, which realizes the hybrid ARQ, can effectively enhance the link performance.
In the Chase combining method, however, the system throughput is not optimized very well because the whole block of data is transmitted repeatedly during each retransmission. In fact, it is unnecessary because the transmitted signals are subjected to the different channel attenuations in a mobile environment. Some parts of signals in the block of data will be transmitted in high signal-to-noise ratios, and other parts will be transmitted in low signal-to-noise ratios. In fact, only the signals being in a poor channel condition need to be retransmitted because the code-error-rate depends always on the signals being attenuated more seriously, if so, the efficiency of the whole system will be increased.